It looks as if the SSD is overloaded. The Netbook went to standby, as I wasn't using it for anything other than sync'ing Bitcoin. When I restarted it, the sync report indicated that it had dropped back over a year, so obviously the disk is unhappy. It's a decent Samsung one, and it hasn't been used all that much, but it is reporting only 47Gb of free space, and it is going to use a lot more than that to get much further. I think I'll buy a new drive for the blockchain.
I like Samsung, and there is quite a lot of choice in 2Tb SSDs. I need to weigh performance, cost and reliability, and my initial decision is to get an 860 QVO internal drive, and insert it into a case for USB connection. As the drive will just be used for the Bitcoin node, price was a consideration. The QVO seems to be slightly inferior to the EVO, but this relates to disk activity. As the blockchain is incremental rather than constantly being moved and replaced, I thought that the cost saving was worth the marginal loss in performance and endurance. Models other than the QVO and EVO are more expensive, and I don't think the improvements are worth the extra costs. I decided to case the drive myself for two reasons. I may want to move it into a notebook at some stage in the future, and there is a cost saving. Transfer speeds may be faster with a factory built external drive, but I don't believe that this will be significant on low to mid spec note/net books. At the moment the netbook is running under Ubuntu, but I may replace this if the machine is going to be used mainly for crypto projects. I'll need to research that.
Any comments or advice will be received gratefully.
