Owners is disabled in disk utility for external hard drive.

I have moved and copied dozens of gigabytes to and from these external hard drives since I bought my mac back in 2013. Same format, no issues whatsoever.

Then you had third-party software installed that allowed you to do that.

Mr_Porter -- see the links I sent earlier and also this one which is from Seagate:

https://www.seagate.com/support/kb/how-to-use-a-backup-plus-or-goflex-drive-with-the-macos-006151en/

It explains that without the special driver installed, you cannot write to the NTFS drive. When you did your "clean install" you lost the system extension (KEXT) that allowed write access to the drives because you did not reinstall it. The clean install removed the KEXT. This link also explains how to reformat the drives if you want them just for file storage or for Time Machine use. Before reformatting them you would need to back up all files of course. I recommend the reformat approach because that NTFS driver is operating system dependent, e.g. it may work for Catalina but may require new versions for future OS updates.

What I was describing needs to be done by an account with Administrator privileges. Check that your account has Administrator privileges (go to Users and Groups in System Preferences and make sure it says "Admin" under your name in the list in the left hand column).

Below is what I see with one of my external drives. After unlocking the padlock, note the area with the tiny up/down arrows next to Read, Read & Write etc. By clicking on those tiny up/down arrows you can change it so everyone can read/write. Some drives, if used for Time Machine backup, may be "owned" by the user who created it, so if that was a different user, that might create an issue. See if you can adjust the privileges with those up/down arrows which should present you with a menu of options.

When you did the fresh install, did you migrate over the same old users from before with Migration Assistant? It sounds like the new user is not recognized as the same as the previous user of those drive, even if the user name might be the same?

In any case, with the drive connected, try Get Info, click the padlock to unlock the window showing drive info, and then if there is a box to "ignore permissions" click that, or change the other permissions to read & write for you and/or all users. Can you do that?

What type of external drives are these? Do you have manufacturer-provided utilities installed for them? What is the format of the drives, HFS+, APFS -- are they GUID partition scheme? How were they originally formatted or partitioned? Are they NTFS or some other Windows type format scheme? What are they used for (Time Machine, general storage, something else)?

Can you see files on these drives? Do you even have read access?

These are 4TB Seagate External hard drives, NFTS format,

macOS cannot natively write to NTFS. It can mount them read-only so you can access the files, but you cannot edit/delete.

You would need third-party software to be able to write to NTFS.

I think the problem is the NTFS format. Manufacturers offer special firmware which is embedded in these drives (or a utility to install and use with these drives) so they can be used with both Macs and PCs. Without the special drivers, firmware, or utilities, the Mac can only read, not write.

See https://www.seagate.com/support/software/paragon/

See also https://www.techwalla.com/articles/how-to-change-permissions-on-an-external-hard-drive-for-a-mac

I suggest that you backup everything on these drives to another drive, then reformat them to native Mac formats (GUID, HFS+, or APFS -- HFS+ should be fine [and might even be preferable] unless you want to use them as bootable drives for Catalina), then restore the contents back to their respective drives.

Alternatively you can download the special software (see above) to fix the problem but it may return again when there is an update to Catalina or when Catalina is replaced with the next OS. I strongly recommend getting rid of NTFS and using native Mac format/partition schemes on all external drives, unless you absolutely must write/read to these drives with both Macs and PCs.

I have moved and copied dozens of gigabytes to and from these external hard drives since I bought my mac back in 2013. Same format, no issues whatsoever.

Then you had third-party software installed that allowed you to do that.