iMac Time Machine first backup to new SSD

HDD only achieves about 80MBs read/write -- internal or external -- so even writing to an SSD is a weak link -- longer if you are encrypting the backup

try AJA System Test Lite app and run it on each SSD, HDD for how the drive is actually performing

if you want the top external speed your 2017 can deliver -- thunderbolt 3 is the way to go:

those numbers are the same on both 2017 and 2019 hardware

I RECOMMEND specifically (pictured):

ACASIS TBU405 Pro 40Gbps M.2 NVMe SSD Enclosure, with Cooling Fan, TBU 405 Pro for M1 M2 Pro/Max, Compatible with USB4/USB3.2/3.1/3.0/2.0, M.2 Enclosure Support 2280/2260/2242/2230

For Data SSD:

WD_BLACK SN850X NVMe Internal Gaming SSD Solid State Drive - Gen4 PCIe, M.2 2280

For boot SSD I would go with a premium SSD like:

SAMSUNG 990 PRO SSD NVMe M.2 PCIe Gen4, M.2 2280

Make sure it has the latest firmware: 5B2QJXD7 (macOS can't flash it - i just sent two of these cards back to Samsung for flashing

older Samsung NVMe SSDs seem to throttle down write speed to 900MBs when used externally so I use them accordingly

the ACASIS enclosure easily swaps M.2 SSDs - an ACASIS enclosure will provide a lot of versatility -- for example, the 2nd enclosure could handle data storage, and swap SSDs to manage TimeMachine and clone systems -- all at blazing speeds

you need to take HDD technology out of your system -- if you are looking for speed -- minimum: swap your internal SATA HDD for an internal SATA SSD (480MBs read/write) -- or get a second external thunderbolt 3 enclosure and use it as your boot volume...

PS: i would also recommend running a Carbon Copy Cloner app "clone" as your redundant system backup, in addition to your timeMachine...

This is totally normal.

Time Machine isn’t a straight file-copy tool, it does a ton of overhead work, scanning every file, comparing versions, creating hard-linked snapshots, and packaging everything into its backup structure.

Your external SSD might be capable of 1000 MB/s, but your 2017 iMac’s 5400-rpm internal HDD can only read at ~70–90 MB/s, and that becomes the bottleneck. On top of that, APFS + Time Machine encryption slows things down even more.

So a 350 GB first backup taking 4+ hours is exactly what I’d expect on that machine. If you want dramatically faster backups, run the iMac from an external SSD or upgrade the internal drive, then Time Machine can actually take advantage of that newer SSD’s speed.

The problem is with the 21.5" iMac's slow 5400rpm internal HDD and not the external 1T SSD.

I'm betting that the Read/Write speed of your iMac's internal 5400rpm HDD is only around 80MB/s.

To check the drive speed of your 21.5" iMac's drive and to look for other problems.

Download and run the free version of EtreCheckPro, from > https://etrecheck.com/en/index.html

Then post back here with your Report, as per > How to use the Add Text Feature When Post… - Apple Community

If you want to speed up that iMac, you can run it from an external SSD.

see > Use an external SSD as your startup disk … - Apple Community

including internal or external HDD (the old spinning style hard drives) in a clone or TimeMachine backup or restore (migration) process will cause substantial slowing of the process(es)

in other words:

the read/write speeds of SSD are a significant time factor in these processes -- like riding a bicycle to work (HDD) verses driving in a car (SSD)

the process will be greatly shortened with SATA SSD, NVMe SSD (solid state drive) hardware -- no HDD in the process

in the past couple years, I've done dozens of clones, TimeMachine backups and restores, migrations -- to observe this behavior -- my working boot volumes typically contain around 600GB on internal NVMe SSD -- if I target my destination to HDD (internal or external) the backup takes overnight -- but only a few hours on NVMe SSD (2000 MBs speed), a little more time on SATA SSD (480 MBs speed)

similar results if my source volume is HDD to SSD destination

HDD technology (80 MBs speed) is a bottleneck -- to enjoying these 2017-2019 (and earlier) hardware -- worth your time and resources to rid your machines of it (except for data storage if you don't need the speed)

claus237 wrote:

"First Time om Time Machine"......or First Time on SSD Harddrive?

Creating a new Time Machine archive on a new fast storage device using a very slow source storage device.

Did you format the SSD to APFS?

Is this typical?

Yes.

If so, why does Time Machine take so long?

The processing "overhead" required by Time Machine. The SSDs speed advantage is utterly irrelevant to it.

"First Time om Time Machine"......or First Time on SSD Harddrive?

I have a 2017 27" iMac., running Ventura 13.7.8, and I still using the old Mechanical Harddrive to my Time Machine back-up!