You’re not missing anything obvious — you’ve done all the right, simple checks (NTFS, Basic, Healthy, BitLocker off). The most likely reason File History won’t accept those H–M partitions is that Windows is treating the physical Seagate enclosure as a “removable/portable” device; when that happens Windows and File History can be picky about using multiple partitions on that single USB device. This is a fairly common real‑world limitation (not a problem with your folder layout or drive health).
So — practical, no‑stress choices you can pick from (ranked by simplicity)
- Easiest (recommended): Use a small, dedicated USB drive for File History for each laptop (or one you move between machines). Buy a cheap 64–128 GB USB stick, set File History to that. No fiddly commands, no partition juggling.
- Low effort alternative: Keep using Macrium for full images (you already do) and copy important personal folders manually to one of the larger partitions on the Seagate when you feel like it. Not automated, but simple and safe.
- If you want automated backups and already use Google Drive: continue using Google Drive to sync your most important folders (Documents, Pictures) and rely on Macrium for full images — that combo avoids File History entirely.
- If later you want me to help (one click, no heavy brainwork): I can show one tiny PowerShell command that proves whether Windows sees the physical disk as removable (so you’ll know for sure). If it is removable and you do want automation, I can also give the effortless step for creating a VHDX on the Seagate and mounting it as a fixed virtual disk (that makes File History happy) — but that is optional and a bit more advanced.
If you’d like the one simple test now (just to confirm), run this in an Administrator PowerShell and paste the single‑line result here:
Get-Disk | Format-Table Number,FriendlyName,BusType,IsRemovable -Auto
(If that’s too much right now, don’t worry — it won’t change anything. Your simplest path is still: buy a small spare USB and point File History to it.
You’ve already done the hard part: partitioning, testing the drive, and keeping images with Macrium. If you want, tell me whether you’d prefer the “buy a small USB” option or the “I’ll do the test later” option, and I’ll give one tiny next step that won’t tax the brain.