Done as you say - very interesting there seems to be a number (hundreds) of references to "FontImporter" failing...
There are many instances of this failure but but 'cut' most of them other than a few examples that look to contain slightly different information:
19/09/13 8:18:10.466 AM com.apple.backupd[42018]: Starting manual backup
19/09/13 8:18:10.631 AM mdworker[41978]: FontImporter: Validation failed - "/Volumes/MEDIA_1TB/Fonts Backup/Fonts/M/Perpetua/Monotype/MPerpetua".
19/09/13 8:18:10.631 AM mdworker[41978]: FontImporter: Validation Result - "(
kATSFontTestSeverityFatalError,
kATSFontTestSeverityInformation
)".
[cut]19/09/13 8:18:30.775 AM sandboxd[42004]: ([42007]) mdworker(42007) deny file-read-data /Volumes/MEDIA_1TB/Fonts Backup/Fonts/S/Stone Sans/Adobe Systems/StoneSanIta/..namedfork/rsrc (import fstype:hfs fsflag:4A09018 flags:240000005E diag:0 uti:com.apple.font-suitcase plugin:/Library/Spotlight/Font.mdimporter - find suspect file using: sudo mdutil -t 153994)
19/09/13 8:18:30.786 AM sandboxd[42004]: ([42007]) mdworker(42007) deny file-read-data /Volumes/MEDIA_1TB/Fonts Backup/Fonts/S/Stone Sans/Adobe Systems/StoneSanIta (import fstype:hfs fsflag:4A09018 flags:240000005E diag:0 uti:com.apple.font-suitcase plugin:/Library/Spotlight/Font.mdimporter - find suspect file using: sudo mdutil -t 153994)
19/09/13 8:18:30.819 AM mdworker[41978]: FontImporter: Validation failed - "/Volumes/MEDIA_1TB/Fonts Backup/Fonts/S/Stop MN/Mecanorma/StopMN t1".
19/09/13 8:18:30.819 AM mdworker[41978]: FontImporter: Validation Result - "(
kATSFontTestSeverityFatalError
)".
[cut]
19/09/13 8:18:39.210 AM sandboxd[42004]: ([41978]) mdworker(41978) deny file-read-data /Volumes/MEDIA_1TB/Fonts Backup/Fonts/W/Wedding Text/Bitstream/WeddiTexBTReg (import fstype:hfs fsflag:4A09018 flags:240000005E diag:0 uti:com.apple.font-suitcase plugin:/Library/Spotlight/Font.mdimporter - find suspect file using: sudo mdutil -t 155010)
19/09/13 8:18:39.265 AM sandboxd[42004]: ([41978]) mdworker(41978) deny file-read-data /Volumes/MEDIA_1TB/Fonts Backup/Fonts/W/Weidemann/International Typeface/WeideBlaIta (import fstype:hfs fsflag:4A09018 flags:240000005E diag:0 uti:com.apple.font-suitcase plugin:/Library/Spotlight/Font.mdimporter - find suspect file using: sudo mdutil -t 155020)
[cut]
19/09/13 8:18:39.330 AM sandboxd[42004]: ([41978]) mdworker(41978) deny file-read-data /Volumes/MEDIA_1TB/Fonts Backup/Fonts/W/Weidemann/International Typeface/WeideBolIta/..namedfork/rsrc (import fstype:hfs fsflag:4A09018 flags:240000005E diag:0 uti:com.apple.font-suitcase plugin:/Library/Spotlight/Font.mdimporter - find suspect file using: sudo mdutil -t 155022)
19/09/13 8:18:39.339 AM sandboxd[42004]: ([41978]) mdworker(41978) deny file-read-data /Volumes/MEDIA_1TB/Fonts Backup/Fonts/W/Weidemann/International Typeface/WeideBolIta (import fstype:hfs fsflag:4A09018 flags:240000005E diag:0 uti:com.apple.font-suitcase plugin:/Library/Spotlight/Font.mdimporter - find suspect file using: sudo mdutil -t 155022)
19/09/13 8:18:39.340 AM mdworker[41978]: FontImporter: Validation failed - "/Volumes/MEDIA_1TB/Fonts Backup/Fonts/W/Weidemann/International Typeface/Weidemann Book".
19/09/13 8:18:39.341 AM mdworker[41978]: FontImporter: Validation Result - "(
kATSFontTestSeverityFatalError
)".
[cut]
19/09/13 8:18:41.214 AM mdworker[41978]: FontImporter: Validation failed - "/Volumes/MEDIA_1TB/Fonts Backup/Fonts/Z/Zymurgy/Unknown/Zymurgy".
19/09/13 8:18:41.215 AM mdworker[41978]: FontImporter: Validation Result - "(
kATSFontTestSeverityFatalError
)".
19/09/13 8:18:44.068 AM mdworker[41978]: FontImporter: Validation failed - "/Volumes/MEDIA_1TB/Fonts Backup/New Fonts-bk/MSO2004-fonts/MT Extra".
19/09/13 8:18:44.068 AM mdworker[41978]: FontImporter: Validation Result - "(
kATSFontTestSeverityInformation,
kATSFontTestSeverityTechnicalError,
kATSFontTestSeverityMajorError,
kATSFontTestSeverityFatalError,
kATSFontTestSeverityMinorError
)".
19/09/13 8:18:51.559 AM com.apple.backupd[42018]: Copied 6127 files (81.7 MB) from volume MacMini i5 HD.
19/09/13 8:18:52.082 AM com.apple.backupd[42018]: Copied 6128 files (81.7 MB) from volume MEDIA_1TB.
19/09/13 8:18:52.097 AM com.apple.backupd[42018]: Using file event preflight for MacMini i5 HD
19/09/13 8:18:52.107 AM com.apple.backupd[42018]: Will copy (Zero KB) from MacMini i5 HD
19/09/13 8:18:52.110 AM com.apple.backupd[42018]: Found 14 files (Zero KB) needing backup
19/09/13 8:18:52.111 AM com.apple.backupd[42018]: 751.9 MB required (including padding), 1.92 TB available
19/09/13 8:18:52.758 AM com.apple.backupd[42018]: Copied 486 files (82 bytes) from volume MacMini i5 HD.
19/09/13 8:18:52.893 AM com.apple.backupd[42018]: Copied 487 files (82 bytes) from volume MEDIA_1TB.
19/09/13 8:18:52.919 AM com.apple.backupd[42018]: Created new backup: 2013-09-19-081852
19/09/13 8:18:53.082 AM com.apple.backupd[42018]: Starting post-backup thinning
19/09/13 8:18:53.082 AM com.apple.backupd[42018]: No post-back up thinning needed: no expired backups exist
19/09/13 8:18:53.402 AM com.apple.backupd[42018]: Backup completed successfully.
I tried 'excluding' that folder from the TM back up and running 'backup now', but looking at the backup folder still nothing from the external 'Media' drive shows up.
And looking back at the Console logs, the same message appear regarding the 'Fontimporter'.