OK, got it working. Turns out that an NTFS volume where the DB files were located got heavily fragmented. Stopped SQL Server, defragmented the whole thing and all it was fine ever since.
If you want to just move the table to a new filegroup, you need to recreate the clustered index on the table (after all: the clustered index is the table data) on the new filegroup you want. You can do this with e.g.: CREATE CLUSTERED INDEX CIX_YourTable ON dbo.YourTable(YourClusteringKeyFields) WITH DROP_EXISTING ON [filegroup_name] or if … Read more