I have made a decision to install Mac OS Big Sur into a used 1TB external HD, which disk has already used over 500GB of files. I left my original internal SSD to stay 10.15.5. It's a safety decision that I believe I could have both systems to play with. Yes, I have a external Mac OS Big Sur beta running cool (a bit slow) on my external HD connected by Type-C cable to my Mac Book Pro. But.... there are nearly 600GB data on my external HD has vanished!!! There are 4 clean normal folders look like a new formatted system disk. There are only 230GB out of 1TB free space left. The disk size is 899.8GB used and 230GB un-used. The old data lost in the "____"? I can find it by Disk Utility. however, there are something wrong, the first partition is dimmed. the second data partition is on. The data partition has a big blue 600GB bar, plus a dark grey small bar before the empty space of 230GB. After that, I switch back to Mac OS 10.15.5. and see the external disk, a Popup saids "unsupportable disk format" and shows a strange disk name called "update". The disk size is 899.8GB used and 230GB un-used. I went "startup disk" to check: it is the system disk with its own disk name on it. (It is not shows "Update") I need to know how to get the data back? Please help.