Zero Bytes after a few seconds

We've been receiving  a strange zero bytes issue when saving files. We are running a few iMacs on the network, it's a mix machines running MacOS Monterey down to High Sierra. 

We can save a variety of files such as PNG, JPEG and adobe AI files. Yet after saving the files, they change the file size to zero bytes. This is regardless of file types and is only happening on new files saves to the  NAS. Old untouched files are not affected.

Likewise, we're experiencing a few filename issues where files are being saved as test1.jpg and a few days later we noticed they had been renamed to something like test1.j or test1.j_ 

This seems to be irrelevant of any disk defrag or scrub as these are completed overnight on a monthly basis. We do not save files to the NAS regularly, maybe a few read/wries per hour.

IPv6 is off

Antivirus is disabled

File Search is disabled.

EXT4 with 1.3TB USED of 2.7TB

87.5GB used for snapshots

4 x 1TB drives

Disk1 Health:

All ok

Disk2 Health:

Current Pending Sector Count: 25

Uncorrectable Sector count: 24

Disk3 Health:

Reallocated sectors: 60

Reallocation events: 55 

Disk4 Health:

ATA Error Count: 56

Reallocated sectors: 11Reallocation events: 12 

Can I safely assume the issue is caused by hard drive 2 and 4 and they need replacing? (but not at the same time as we are using RAID5)

I also noted that when I check the file info on a zero bytes file, there is an issue with one of the users (this spinning circle appears on teh file info on all files)

I have given my colleague a separate user account with full privelidges as he uses it the most and was concerned it was a corrupt account. But this did not change anything.

Any advice would be appreciated.

Thanks in advance