This isn't a big deal, but it seems idl7 doesn't like setting hdf compression, if a dimension is unlimited.  IDL Version 7.0.1 (linux x86_64 m64). (c) 2008, ITT Visual Information Solutions .... IDL> SDinterface_id = HDF_SD_START('compress.hdf', /CREATE) % Loaded DLM: HDF. IDL> SDdataset_id = HDF_SD_CREATE(SDinterface_id, 'dataset1', [9,0], /long) IDL> HDF_SD_SETCOMPRESS,SDdataset_id,4,EFFORT=7 % HDF_SD_SETCOMPRESS: Unable to set compression. % Execution halted at: $MAIN$ IDL> SDdataset_id = HDF_SD_CREATE(SDinterface_id, 'dataset1', [9,10], /long) IDL> HDF_SD_SETCOMPRESS,SDdataset_id,4,EFFORT=7 IDL>   I am a very infrequent user of IDL.  I am running code was previously run on IDL 6.3, but I only have access to IDL 7 now.   hdf_sd_setcompress worked before with the dimension set to 0. (for version 6.3).  For hdf_sd_create, "If an UNLIMITED dimension is desired, set the last vector element to zero or a negative number." I just thougth I would mention it.  I'll try to figure out why the dimensions weren't specifed explicitly.