What is the NBU file format?
.nbu (Nokia Back Up) is the extension used by files created with Nokia PC Suite - and later Nokia Ovi Suite and Nokia Suite - a software tool for creating full data backup copies of Nokia phones. .nbu files contain a backup copy of such data as contacts, SMS messages, calendars, notes, bookmarks, and media files.
.nbu backup files are used for restoring backed-up phone data to a compatible Nokia handset. .nbu files can also be used for transferring data to a different phone. Nokia Suite was discontinued in 2014, making .nbu a legacy format; the free open-source NbuExplorer is now the primary tool for opening and extracting content, exporting sections to formats such as .vcf contacts and .vmg messages. Unlike the partial .nfb backup, .nbu is Nokia's most complete backup type, capturing all supported phone data in a single proprietary binary container.
Security & safety
RISK: LOWAn NBU is passive backup data, not executable code, so opening one is low-risk. The real concern is PRIVACY: a backup contains personal contacts, private messages and photos, so handle it carefully and don't upload it to untrusted 'online NBU viewer' sites. NbuExplorer is open-source and works fully offline - prefer it. Beware paid 'Nokia backup recovery' tools that mostly wrap the same free extraction; they are unnecessary for a non-corrupt NBU.
Format details
in a nutshellPrograms that open NBU files
Windows2 apps
NbuExplorer
Open-source Open the .nbu; browse contacts/messages/notes/media and Extract - exports contacts to .vcf, SMS to .vmg, notes to .txt. The practical opener today; also reads .nfb/.nfc/.nbf/.arc. 
Nokia Suite (legacy)
Free Nokia's own Restore tool - historically the way to restore an NBU back onto a Nokia phone. Discontinued in 2014 and hard to install on modern Windows; use only with a legacy setup. Technical details
deep spec| Format type | Proprietary binary backup container (not ZIP or any standard archive format) |
| Encoding | Binary, little-endian internal fields |
| Backup scope | Full device backup: contacts, SMS/MMS, calendar, notes, bookmarks, settings, and media files |
| Target platform | Nokia Symbian (S60) and Series 40 (S40) handsets |
| Creating software | Nokia PC Suite, Nokia Ovi Suite, Nokia Suite (Windows-only desktop applications) |
| Compression | None - data blocks stored largely uncompressed |
| Encryption | None - contents are unencrypted within the container |
| File signature | No fixed public magic bytes; structure is undocumented and parsed heuristically by third-party tools |
| MIME type | application/octet-stream |
| Typical file size | A few MB to several GB, depending on the volume of photos, videos, and messages |
| Active era | Approximately 2004 (Nokia PC Suite 6.x) through 2014 (Nokia Suite discontinuation) |
| Related Nokia backup types | .nfb (partial/selective backup), .nfc (contacts and messages only), .nbf, .arc |
| Export formats via NbuExplorer | .vcf (contacts), .vmg (messages), .vcs (calendar), plain text (notes) |
| Modern compatibility | Legacy format; readable today only via NbuExplorer (free, open-source, Windows) - Nokia Suite is no longer functional on modern Windows |
| Released | around 2004 (Nokia PC Suite); used through Nokia Ovi Suite and Nokia Suite (to ~2014) |
| Latest version | Nokia Suite-era NBU (no public version after Nokia Suite was discontinued in 2014) |
NBU conversions
Community Q&A
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