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iPad causing external SSD mounting errors

Hi,


I’m trying to connect my external SSD to my iPad Pro. It is SanDisk Extreme Portable SSD 2TB USB-C and in exFat format. The first time I connect it to my iPad Pro 2020, it is mounted and the files can be read. However, when I plug it out and connect it again, it doesn't show up in the list.


In order to solve this problem every time, I connect it to my Windows PC and let Windows repair my SSD (it tells me erros are found on the device).


How can I solve this problem? Is there a way to troubleshoot external SSDs on iPad and repair it like on Windows? Any idea on why is the iPad causing errors on the external SSD? It is really annoying to repair it on Windows first before I can use it again with my iPad.


Thanks.

iPad Pro 12.9-inch Wi-Fi

Posted on May 4, 2020 2:09 AM

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Question marked as Top-ranking reply

Posted on May 4, 2020 4:00 AM

The technical cause is likely to be write-caching - this being a feature of the USB Storage (in this case SSD) device.


For most desktop computers, it is possible to configure an externally attached storage device either for performance (i.e., uses write-caching) or for Quick Removal. This determines how and when information is “written” to non-volatile storage.


When configured for Quick Removal, information from the host computer is written immediately to the storage medium. This has the advantage that you may disconnect the storage device from the host computer, without risk of corrupting the file-system, at any time that data is not being actively transmitted to the storage device. Whilst being a completely valid and useful method of managing data storage, for reasons beyond the immediate scope of this description, this method is less efficient and slower.


Alternatively, when write-caching is enabled, information from the host computer is “cached” in separate (high performance) volatile storage within the storage device - and when sufficient information has been received, a “block” of data is written in single write-cycle. Whilst being faster and more efficient, this method comes with the penalty of the host computer having to notify the storage device of a impending device-disconnection before it is disconnected. This warning ensures that the storage device will flush any pending data from its volatile cache-memory to non-volatile storage - and in so doing sets a “clean switch” on the flash filesystem and signals to the computer that it is safe to disconnect.


A drive configured for write-caching, upon connection to a host computer, is checked for the “clean switch”; if present, the storage device is “mounted” and made available to the operating system. By contrast, if the “clean switch” is not detected (this will occur if the storage device was disconnected prior to being notified of a “dismount”), the filesystem must be assumed to be potentially corrupt; pending data within the write-cache may not have been written to the drive.


So, the relevance of this to iPad is simple. iPad does not fully support devices that are implement write-caching. iPadOS lacks the ability to inform the storage device of imminent disconnection of the storage device from the USB bus. iPad also lacks capability to detect and scan/repair corrupt storage devices.


In summary, now that you [hopefully] understand both the cause and effect, you will now understand that you can only reliably use USB storage devices that can be configured for Quick Disconnect operation - this having major significance to non-corruption of the filesystem and reduced data throughput.


I hope this solves your mystery - and proves to be helpful in optimising your SSD device for use with your iPad.

3 replies
Question marked as Top-ranking reply

May 4, 2020 4:00 AM in response to joehashisho

The technical cause is likely to be write-caching - this being a feature of the USB Storage (in this case SSD) device.


For most desktop computers, it is possible to configure an externally attached storage device either for performance (i.e., uses write-caching) or for Quick Removal. This determines how and when information is “written” to non-volatile storage.


When configured for Quick Removal, information from the host computer is written immediately to the storage medium. This has the advantage that you may disconnect the storage device from the host computer, without risk of corrupting the file-system, at any time that data is not being actively transmitted to the storage device. Whilst being a completely valid and useful method of managing data storage, for reasons beyond the immediate scope of this description, this method is less efficient and slower.


Alternatively, when write-caching is enabled, information from the host computer is “cached” in separate (high performance) volatile storage within the storage device - and when sufficient information has been received, a “block” of data is written in single write-cycle. Whilst being faster and more efficient, this method comes with the penalty of the host computer having to notify the storage device of a impending device-disconnection before it is disconnected. This warning ensures that the storage device will flush any pending data from its volatile cache-memory to non-volatile storage - and in so doing sets a “clean switch” on the flash filesystem and signals to the computer that it is safe to disconnect.


A drive configured for write-caching, upon connection to a host computer, is checked for the “clean switch”; if present, the storage device is “mounted” and made available to the operating system. By contrast, if the “clean switch” is not detected (this will occur if the storage device was disconnected prior to being notified of a “dismount”), the filesystem must be assumed to be potentially corrupt; pending data within the write-cache may not have been written to the drive.


So, the relevance of this to iPad is simple. iPad does not fully support devices that are implement write-caching. iPadOS lacks the ability to inform the storage device of imminent disconnection of the storage device from the USB bus. iPad also lacks capability to detect and scan/repair corrupt storage devices.


In summary, now that you [hopefully] understand both the cause and effect, you will now understand that you can only reliably use USB storage devices that can be configured for Quick Disconnect operation - this having major significance to non-corruption of the filesystem and reduced data throughput.


I hope this solves your mystery - and proves to be helpful in optimising your SSD device for use with your iPad.

May 4, 2020 9:10 AM in response to LotusPilot

Great explanation! I had a feeling that disconnecting the storage device from the iPad without a prior notification would cause this problem. I tried as well turning off the iPad before disconnecting the storage device as a workaround to avoid this problem but didn‘t work out.


Now that I know exactly what the problem is and what’s causing it, I will try to optimize my SSD using the information above.


Thanks a lot for your help!

May 4, 2020 9:24 AM in response to joehashisho

You’re very welcome. With luck, you’ll be able to use your newfound knowledge of SSD to some useful effect.


Mounting external storage devices on iPad is more complicated than many anticipate. The majority of iPad users have an innate expectation of a simple “it just works” paradigm - and, in context of external storage, have little (to no) understanding of formatting, partitioning or other technical considerations.


The functional enhancements introduced in iPadOS make the platform much more useful to advanced users - but with a consequent reduction in simplicity that often presents unexpected difficulties.



iPad causing external SSD mounting errors

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