All sorted, new install of OS, updated, installed contribs, broke existing raid, copied ~1Tb data from old raid to new degraded raid, installed new OS drive, mounted new data raid and re-mirrored using remaining old raid drive. Total down time ~10 minutes, several reboots, mainly hardware cabling, finger problems and confirming changes. Most of the down time was during the running of migratehelper while it stopped services which can't be helped. Everything I needed to work out was easily found on google and here.