With storage disk, assuming it is the new disk, did you check “format” (it will erase everything on it)?
Hi Boris, what I see so far is seems there is some kind of incompatibility between the disk formatting on the Syncloud vs my Linux PC. The Syncloud formatted disk cannot mount on the PC and the PC formatted disk cannot mount on the Syncloud system. Seems the complaints are very similar on both sides: something about Partition Table and may be MBR. I have not dig further to understand what exactly they do not like.
I am planning to try now formatting the new disk on the Syncloud system and see how it will go and if it will work also on my PC after that. Unfortunately I will loose all the data I copied from the original Syncloud disk which took me over 12 hours. Anyway I am in process of working on that plan. Hope after that I will be able to restore the original data onto the new HD. I will keep you posted on my adventures. 
Ideally you have a third disk on your PC for a backup on case there is some problem/mistake while copying formatting.
Also if you can attach two disks to Syncloud device you could format the new one then manually mount the old one and copy faster.
Thank you Boris for the advise. Yes I have in fact two backups of my data but that is the actual data including the files that are in Nextcloud but that is not the Synclud disk itself. May be I should start backing it up too.
Seems I finally got things back to working state and even managed to upgrade the Nextcloud App. Yay! What I found in the process is that Syncloud does not like anything larger than 2TB. My new disk is 4TB and that is the reason it could not mount (add) it on the Syncloud. I tried configuring it as disk – did not work, tried as partition with single 4TB partition – did not like it. The only configuration that I found works is if the disk is partitioned with at least one partition of up to 2TB in size. I am a bit puzzled by that fact because I did partition and format the new disk on the Syncloud box using ‘fdisk’ and I was able to format the whole 4TB without any issues. Though when I try to add it either as ‘disk’ or ‘partition’ it will not work if the partition is larger than 2TB. In all those tries I had been selecting the Syncloud to format it and always it will error out if the size is more than 2TB.
Is that a system limitation or on purpose by design? Or maybe some kind of misconfiguration on my side? If it is misconfiguration, how can I fix it?
I will get back tou you, created an issue storage disk size limitation · Issue #726 · syncloud/platform · GitHub
Is there any log, ideally you should send logs as soon as you have an error message?
I will need to buy a big disk for testing.
I thought in disk mode we use btrfs directly without partition table which usually brings the limitation if it is not GPT, let me check.
strange both partition and disk modes should not have the limitation, it would be very helpful if you could activate a spare 4tb disk partition and disk with format and send logs, if not I will test this when I get 4tb disk myself.
Hi Boris, thank you for looking into that. Unfortunately I did not save any logs while experimenting with the 4TB drive. Right now I do not have extra time to spend on this issue. Hope you will be able to figure out what is going on soon. I am sorry I cannot be of assistance at the moment.