Is it possible to determine NAS allocation unit size or to recover lost NAS allocation units? Chkdsk will perform those tasks and more for local Windows 10 drives, but it appears that the only diagnostic tool available for NAS units is the S.M.A.R.T utility, which for my NAS took about 15.5 hours to complete a scan of a 4 TB NAS and report that no errors were found even though there's about 0.5 TB of unused space that isn't available for use. My NAS has a separate apparently proprietary Scan disk utility. That scan took about 3.5 hours and also reported no errors.
 
I contacted Synology, the manufacture I'm leaning toward, and was told that Synology uses 4kb allocation units. I followed up asking if Samba roundup could cause their NAS units to allocate more than one allocation unit to all files but don't have any answer to that question so far. I learned about Smaba roundup on the tom'sHardware forum, I think.
 
I have a TRENDnet TN-200 RAID 1 NAS with an apparent allocation unit size of 1,048,576 bytes. I say apparent because I have not found a way to view disk properties and I've read about Samba roundup causing NAS drives to use several allocation units for any file no matter how small. I determined the apparent allocation unit size by using Windows 10's File Explorer > Properties for a ProChannel preset file which is 1,664 bytes and occupies 1,048,576 bytes on the NAS.