There's something wrong on the external drive. You're getting a weird failure, and even aside from that it looks like you may be getting stalls of multiple seconds as Zotero checks attachment files (though it's a little hard to tell from the debug output alone).
I'm afraid I don't really have much to suggest. The fact that this works fine on the SSD, and sometimes works on the HDD, suggests that Zotero itself is working fine and something is causing the HDD access to stall sometimes.
Upgrade Storage
D1855496975
D56497045
Debug ID D938276286
I'm afraid I don't really have much to suggest. The fact that this works fine on the SSD, and sometimes works on the HDD, suggests that Zotero itself is working fine and something is causing the HDD access to stall sometimes.