After about 60% transfer drops to 0 and after that i is copying with minimal transfer

I use TrueNAS in my company as a backup server, which runs backups at night. Generally, I haven’t noticed any problems with it. Last week, I needed to restore a database for testing. Due to the required disk space for the tests, I copied the database to a shared resource provided by TrueNAS where backups are stored. The copying process went smoothly and quickly up to about 60%, but after that, it dropped to 0 and stayed there for a while. After some time, the copying continued but at a very low speed, just a few percent of normal speed. On TrueNAS, I noticed that an alert appeared: Device /dev/gptid/4f6f6573-822f-11ee-a567-9457a567c2c0 is causing slow I/O on pool VD. What could be the problem?
Spec:
HP DL380 G8
2 x Xeon E5-2690
64 GB ram
LSI SAS9211-8i IT Mode
Disks

Blockquote
|HITACHI|HUS72403CLAR3000|C370|
|—|—|—|
|HGST|HUS724030ALS640|A124|
|HGST|HUS724030ALS640|A124|
|HITACHI|HUS72403CLAR3000|C370|
|HITACHI|HUS72403CLAR3000|C370|
|HGST|HUS724030ALS640|A124|
|HITACHI|HUS72403CLAR3000|C370|
|SEAGATE|ST33000650SS|RS11|
|HGST|HUS724030ALS640|A124|
|SEAGATE|ST33000650SS|RS11|
|SEAGATE|ST3000NXCLAR3000|GS1F|
|SEAGATE|ST3000NXCLAR3000|GS1F|