I only just got the TS4K a few days ago.
After getting a USB a to c adapter I tried plugging a SanDisk 128GB thumb drive I had into it. It didn't give a notification when I plugged it in, but it was there when I went to Storage. So I did Format as internal. After running for awhile it just exited to the main screen. However it then only gave me the 'drive has been safely ejected' loop over and over. Rebooting made no difference.
Put the SanDisk drive into my Windows PC, it showed as two drives/partitions (one large, one small) and that both needed formatting to be read. The smaller drive wouldn't format but the big one did, as exFAT. Still when put back into the TS4K it would again only show the 'drive has been safely ejected' message.
Putting it back into my Windows PC I opened Computer > Manage > Disk Management. I deleted both partitions, created one big new partition using the whole drive, then I formatted it as exFAT (NOT using Quick Format), and put it in the TS4K.
This time it worked! It notified me immediately that the SanDisk had been detected, so I formatted it as internal storage, transferred apps to it etc.
I think the first time I inserted the thumb drive into the TS4K it was still formatted as NTFS. This allowed the TiVo to see it, but then led to a bunch of corrupt formatting.
If you have a thumb drive that TS4K messed up and Windows won't read or format it, use Disk Management to delete & recreate it's partition. And don't put an NTFS drive into a TS4K, make sure it's formatted as exFAT first.